For optical fibers, the index profile is generally described as the graphical appearance of the function that associates the refractive index with the radius of the fiber. Conventionally, the distance (r) to the center of the fiber is represented on the abscissa and the difference between the refractive index and the refractive index of the fiber cladding is represented on the ordinate. The index profile is thereby termed as a “step,” “trapezium,” or “triangle” for graphs that have the shapes of a step, a trapezium, or a triangle, respectively. These curves are generally representative of the theoretical or set profile of the fiber. Constraints in manufacturing the fiber, however, may lead to a slightly different profile.
An optical fiber conventionally includes an optical core, which has the purpose of transmitting and possibly amplifying an optical signal, and an outer optical cladding, which has the purpose of confining the optical signal in the core. Accordingly, the refractive index of the core (nc) is greater than the refractive index of the outer optical cladding (ng) (i.e., nc>ng). As is well known, propagation of an optical signal in a single-mode optical fiber is broken down into (i) a fundamental guided mode in the core and (ii) secondary modes guided over a certain distance in the core-cladding assembly, called cladding modes.
Conventionally, step-index fibers, also called single mode fibers (SMFs), are used as a line fiber for transmission systems with optical fibers. These fibers exhibit chromatic dispersion and a chromatic dispersion slope meeting specific telecommunications standards.
For requirements of compatibility between optical systems from different manufacturers, the International Telecommunication Union (ITU) defined a standard with a specification referenced as ITU-T G.652, with which an optical fiber for standard transmission (i.e., a standard single mode fiber or SSMF) should comply.
This specification G.652 recommends, among other things, that for a transmission fiber, the range [8.6-9.5 μm] for the value of the mode field diameter (MFD), at a wavelength of 1310 nm; a maximum of 1260 nm for the value of the cable cut-off wavelength; the range [1300-1324 nm] for the zero dispersion wavelength, noted as λ0; a maximum of 0.092 ps/nm2-km for the value of the chromatic dispersion slope. The cable cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode after propagation over 22 meters of fiber, such as defined by the subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard.
Moreover, for a given fiber, a so-called MAC value is defined as the ratio of the mode field diameter of the fiber at 1550 nm) and the effective cut-off wavelength (λCeff). The effective cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode after propagation over two meters of fiber, as defined by the sub-committee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard. MAC is a parameter for evaluating the fiber performance and is particularly useful for finding a compromise between the mode field diameter, the effective cut-off wavelength, and the bending losses.
FIG. 1 depicts experimental results that illustrate bending losses at a wavelength of 1625 nm with a bending radius of 15 mm in a step-index SSMF versus the value of MAC at the wavelength of 1550 nm. It is seen that the value of MAC influences the bending losses of the fiber and that these bending losses may be reduced by reducing MAC.
Now, a reduction in MAC, by reducing the mode field diameter and/or by increasing the effective cut-off wavelength, may lead away from the G.652 standard and make the fiber commercially incompatible with certain transmission systems.
Indeed, the value of the effective cut-off wavelength (λCeff) cannot be increased beyond a limiting value in order to observe the maximum value of 1260 nm for the cable cut-off frequency (λCC). Furthermore, the value of the mode field diameter for a given wavelength is strictly imposed in order to minimize coupling losses between the fibers.
Reduction of the MAC criterion for limiting the bending losses should therefore be combined with a limitation of the value of the effective cut-off wavelength (λCeff) in order to limit propagation of modes of higher orders in the fiber, while retaining a sufficient mode field diameter to provide coupling without excessive optical losses.
In particular, there is a trade-off between compliance with the G.652 standard and reduction of the bending losses for fibers intended for fiber optical systems up to the individual, so-called “fibers to the home” (FTTH) or fiber optical systems up to the curb or up to the building, so-called “fibers to the curb” (FTTC).
Indeed, a transmission system through optical fibers comprises storage boxes in which fiber overlengths are provided in the case of future interventions. These overlengths are wound in the boxes. Because of the intention to miniaturize these boxes for FTTH or FTTC applications, the single mode fibers in this context are intended to be wound on increasingly small diameters (so as to reach bending radii as small as 15 mm). Moreover, within the scope of FTTH or FTTC applications, the fiber risks being subject to harsher installation constraints than in applications at longer distances (i.e., the presence of accidental bendings related to the installation and to the environment). Provision must be made for the presence of accidental bending radii equal to 7.5 mm or even 5 mm. It is therefore absolutely necessary in order to meet the constraints related to the storage boxes and to the installation constraints that the single mode fibers used for FTTH or FTTC applications have limited bending losses. Nevertheless it is understood that this reduction in bending losses should not be achieved to the detriment of a loss of the single mode character of the signal. This would strongly deteriorate the signal or detrimentally introduce significant junction optical losses.
U.S. Pat. No. 4,852,968 (Reed et al.) relates to a single mode optical fiber having an index profile consisting of a core region, a first cladding region, a trench region, and a second cladding region and, optionally, a second trench region. This patent further relates to the optimization of dispersion characteristics.
In order to obtain optical fibers that meet the requirement for a reduction of bending losses, three kinds of solutions have been proposed in the prior art.
A first solution found in the prior art consists of producing conventional step-index fibers with a reduced mode field diameter. The bending losses are indeed reduced by decreasing MAC because of the reduction in mode field diameter, and a single mode character is retained with a cable cut-off wavelength, which remains less than 1260 nm. Nevertheless, such fibers have significant coupling losses and are not adapted to the FTTH applications described previously.
The publication of I. Sakabe et al., “Enhanced Bending Loss Insensitive Fiber and New Cables for CWDM Access Networks”, Proceeding 53rd IWCS, pp. 112-118 (2004), suggests reducing the mode field diameter of the fiber in order to reduce the bending losses. This reduction in mode field diameter, however, leads away from the G.652 standard.
The publication of T. Yokokawa et al., “Ultra-Low Loss and Bend Insensitive Pure-Silica-Core Fiber Complying with G.652 C/D and its Applications to a Loose Tube Cable,” Proceedings 53rd IWCS, pp. 150-155 (2004), proposes a pure silica core fiber (PSCF) having reduced transmission and bending losses, but with a reduced mode field diameter that is located outside the G.652 standard.
A second solution found in the prior art consists of producing step-index fibers with a depressed section (i.e., a central core, an intermediate cladding, and a depressed cladding). With such a structure, it is possible to actually reduce the bending losses with constant MAC for small bending radii, typically 10 mm.
The publications of S. Matsuo et al., “Low-Bending-Loss and Low-Splice-Loss Single-Mode Fibers Employing a Trench Index Profile,” Journal of Lightwave Technology, Vol. 23 no. 11, pp. 3494-3499 (2005), and K. Himeno et al., “Low-Bending-Loss Single Mode Fibers For Fiber-To-The Home,” Journal of Lightwave Technology, Vol. 23, No. 11, pp. 3494-3499, (2005), propose such fiber structures with depressed section in order to reduce the bending losses.
Analyses, however, demonstrate that if the bending losses may be substantially improved with a fiber profile with depressed sections, such a profile also causes an increase in the effective cut-off wavelength by the occurrence of resistant leakage modes, which mainly propagate in the intermediate cladding and the depressed section.
Consequently, this requires that fibers having a MAC less than 7.9 at 1550 nm be selected in order to compensate the fact that the effective cut-off wavelength is much higher than expected while guaranteeing bending losses at 1625 nm less than 0.1 dB/turn for a winding with a radius of 15 mm; whereas for a SSMF, it is sufficient to select fibers with a MAC less than 8.1 at 1550 nm in order to guarantee losses less than 0.1 dB/turn at 1625 nm for a bending radius of 15 mm (from the results of FIG. 1). The manufacturing yield for such step-index fibers with a depressed section is therefore reduced.
A third solution found in the prior art consists of producing hole-assisted step-index fibers.
The publication of K. Bandou et al., “Development of Premise Optical Wiring Components Using Hole-Assisted Fiber,” Proceedings 53rd IWCS, pp. 119-122 (2004), proposes a fiber with holes, the fiber having the optical characteristics of a step-index SSMF with reduced bending losses.
Such hole-assisted step-index fibers for reducing the bending losses have also been described in the publications of T. Hasegawa et al., “Bend-Insensitive Single-Mode Holey Fiber with SMF-Compatibility for Optical Wiring Applications,” in Proceedings ECOC'03, paper We2.7.3, Rimini, Italy (2003); of D. Nishioka et al., “Development of Holey Fiber Supporting Extra-Small Diameter Bending,” SEI Technical Review, No. 58, pp. 42-47 (2004); of K. Miyake et al., “Bend Resistant Photonic Crystal Fiber Compatible with Conventional Single Mode Fiber,” in Proceedings ECOC'04, paper Mo3.3.4, Stockholm, Sweden (2004); of Y. Tsuchida et al., “Design and Characterization of Single-Mode Holey Fibers with Low Bending Losses”, Optics Express, Vol. 13, No. 12, pp. 4470-4479 (2005); of K. Ohsono et al., “High Performance Optical Fibers for Next Generation Transmission Systems,” Hitachi Cable Review, No. 22, pp. 1-5, (2003); of K. Nakajima et al., “Hole-Assisted Fiber Design for Small Bending and Splice Loss,” IEEE Photonics Technology Letters, Vol. 15, No. 12, pp. 1737-1739, (2003); of K. Ieda et al., “Transmission Characteristics of a Hole-Assisted Fiber Cord for Flexible Optical Wiring,” Proceedings 54th IWCS, pp. 63-68 (2005); of N. Guan et al., “Hole-Assisted Single Mode Fibers for Low Bending Loss,” in Proceedings ECOC'04, paper Mo3.3.5, Stockholm, Sweden (2004), and of K. Himeno et al., “Low-Bending-Loss Single-Mode Fibers for Fiber-To-The-Home”, Journal of Lightwave Technology, Vol. 23, No. 11, pp. 3494-3499 (2005).
The cost for manufacturing such a fiber and the presently high attenuation levels (>0.25 dB/km), however, make commercial use in FTTH or FTTC systems impractical. Further, with these fibers, it is simply not possible to achieve the optical characteristics recommended by the G.652 standard, notably in terms of chromatic dispersion.
Therefore, there is a need for a transmission fiber with which it is possible to meet the criteria of the G.652 standard (i.e., commercially usable in FTTH or FTTC transmission systems) and which has reduced bending and microbending losses. In particular, there is a need for a fiber that has reduced losses for a bending radius of 15 mm and also for a bending radius as small as 7.5 mm. Indeed, in FTTH applications, overlengths of fibers are generally wound in increasingly miniaturized storage boxes. Moreover, the fiber will be subject to significant bending stresses related to the environment of its installation.